Where Boros Charm has really only two functions, protecting and dealing damage, Simic charm can protect just as well, if not better, and damage almost just as well. In addition to that, Simic charm can bounce, which seems to push this one to the better end of the spectrum. This is not to say that Boros Charm is bad, but maybe that simic charm is underrated, if only slightly.

It's not really comparable to Unsummon/Giant Growth - it's just far too versatile. What good is unsummon when you're 3 damage away from winning or when your opponent is about to destroy your best creature? (okay it can bounce the creature but wouldn't you rather keep it on the field?) What good is giant growth when none of your attacks can get through because your opponent has some massive blocker or something like Fog bank or when an important artifact is about to be Smelted?

This suits nicely with the colors of the guild. Pumping would save your creatures or deliver a decisive blow, hexproof on all PERMANENTS avoid spells, and unsommon effect not only remove enemy creatures it also allow you to preserve yours, or benefit from recasting those creatures so you can trigger evolve or other comes-into-play effects,